This invention relates to devices for repairing fractures of the hip joint, and, more particularly, to a compression hip screw which applies a continuous compression force to a fracture between the head and neck of the femur.
The hip joint is one of the most heavily stressed load-carrying skeletal joints. It is essentially a ball-and-socket joint formed by the head of the femur which pivots within the cup-shaped acetabulum of the pelvis. A common problem with patients having severe arthritic conditions or osteoporosis is the occurrence of a fracture at the neck of the femur between the femoral head and its proximal end caused by an external trauma or deterioration of the bone. Unless the femoral head and neck have become so porous and weakened by osteoporosis or other conditions as to require complete replacement of the hip joint, it is preferable to reduce the fracture and induce healing.
One early surgical procedure used to repair fractures in the area of the femoral head was an autologus bone graft in which healthy bone is implanted in the area of the fracture to replace the damaged bone. A problem with this procedure is the necessity for opening a second surgical site to remove healthy bone for placement into the hip joint.
Beginning in the 1950's, compression hip screws were developed as an alternative to autologus bone grafts for the treatment of fractures at the neck and head of the femur. Early compression hip screws comprised a lag screw having external threads at one end and a threaded bore at the opposite end, a side plate formed with a shank section connected to a hollow barrel and a compression screw for connecting the lag screw to the side plate. To implant the early compression hip screws, the lag screw is first inserted through the femoral neck so that its externally threaded end enters into the dense cancellous bone of the femoral head, and the opposite, outer end extends into the neck of the femur. The next step in the surgical procedure is to mount the shank of the side plate to the lateral aspect of the femur so that the hollow barrel extends over the outer end of the lag screw. The compression screw is then inserted through the barrel and into engagement with the threaded bore of the lag screw. The head of the compression screw seats within a recess formed at the outer end of the barrel so that when tightened, the compression screw urges the lag screw toward the side plate. In turn, the femoral head is drawn by the lag screw toward the neck of the femur so that a compression force is applied to the fracture, which aids in healing. The lag screw and compression screw are axially slidable within the hollow barrel to permit movement with the femoral head if the fracture settles or the bone further deteriorates. This prevents the externally threaded end of the lag screw from piercing through the femoral head and into the cartilage of the acetabulum which could happen if the femoral head should settle to any significant degree onto the neck of the femur and the lag screw was held in a fixed axial position.
One of the problems with such early compression hip screws is that the lag screw was permitted not only to move axially within the hollow barrel but also to rotate. Rotation of the lag screw may result from walking or other activities in which load is applied to the hip joint. It was found that the lag screws of prior art hip screws could be rotated to such an extent that they would either pierce the femoral head and move into the cartilage near the acetabulum, or move in the opposite direction into the soft tissue of the leg. In either position, the lag screws were painful to the patient and required another operation to either remove or replace the entire compression hip screw.
Compression hip screws in most common use today are nearly identical to the above-described original design, except for the addition of elements to prevent rotation of the lag screw within the barrel of the side plate. Typically, the interior of the hollow barrel in conventional, modern compression hip screws is formed with a projection or key which is adapted to mate with a groove or keyway formed in the outer wall of the lag screw. The groove in the lag screw is aligned with the projection in the barrel during the surgical procedure so that when the lag screw is inserted within the barrel, the projection mates with the groove to prevent rotation of the lag screw relative to the fixed hollow barrel. Axial movement of the lag screw within the barrel is permitted, with the projection sliding along the length of the groove.
One problem with the design of modern compression hip screws is that accurate alignment must be achieved between the lag screw and side plate in order for the groove in the lag screw to align with the projection in the barrel of the side plate. As described above, the lag screw is inserted first during the surgical procedure, and thereafter the side plate is positioned so that the barrel fits over the lag screw. Preferably, the lag screw is inserted within the densest bone of the femoral head which is found near the subchondral plate at the acetabulum. If the projection in the barrel and groove in the lag screw do not align, the lag screw must be rotated to the proper position. The surgeon must be careful not to rotate the lag screw further into the femoral head if it is already near the subchondral plate because it could enter the cartilage within the acetabulum. On the other hand, if the lag screw is rotated in the opposite direction for alignment purposes it could become loosened. Often, the lag screw is either extended too deeply into the femoral head or not deeply enough in order to achieve the proper alignment between the lag screw and the barrel of the side plate.
Another important limitation of currently used compression hip screws is their inability to maintain compression at the fracture site if the fracture should settle; that is, if the femoral head settles onto the neck of the femur as the patient puts weight on the hip or as the bone deteriorates due to osteoporosis. As described above, in currently used, conventional hip screws the compression screw is inserted through the barrel of the side plate into the threaded bore of the lag screw and seats within a recess formed in the barrel of the side plate. As the compression screw is tightened, the lag screw is pulled toward the side plate and thus urges the femoral head toward the neck of the femur to apply compression at the fracture site. In order to accommodate settling of the fracture, wherein the femoral head moves toward the neck of the femur, the lag screw and compression screw are axially movable within the barrel toward the side plate. However, once the head of the compression screw is unseated from the recess in the barrel of the side plate, essentially all of the compression applied to the fracture is immediately dissipated.
Studies have shown that healing of any bone fracture is enhanced by the constant application of a compression force across the fracture, which also helps to prevent non-union of the bone while the patient is bedridden. It is common, particularly in elderly patients, for the fractured area of the femoral neck and head to collapse before it has fully healed. In order to guard against such collapse, and to lessen the likelihood of a second fracture at the same location at some future time, compression hip screws are often left in the patient permanently so long as they are not painful. Although known compression hip screws provide some additional support for the hip joint when left in place, their failure to maintain compression at the original fracture site can delay union of the bone.
One prior art compression hip screw design intended to provide a continuous compression force at the fracture site is disclosed in the article, "The Treatment of Displaced Fractures of the Neck of the Femur by Compression", by J. Charnley et al. The Charnley device comprises a lag screw which is threaded at one end for insertion into the femoral head, and also includes external threads at the opposite end. A side plate mounted to the lateral aspect of the femur is formed with a barrel which is adapted to receive the opposite, threaded end of the lag screw. The lag screw is first threaded into the femoral head, and the side plate is then secured to the lateral aspect of the femur so that the outer, threaded end of the lag screw extends within the barrel of the side plate. A compression spring is placed over the outer, threaded end of the lag screw within the barrel, and a nut is then threaded onto such outer end and into contact with the spring. By tightening the nut, the spring is compressed within the barrel, which in turn, exerts a force against the nut urging it in the opposite direction toward the side plate. Since the nut is threaded onto the lag screw, the lag screw is also urged toward the side plate by the spring.
The lag screw, and the nut threaded therealong, are axially movable within the barrel to accommodate settling of the fracture as in other prior art compression hip screws. However, in the Charnley hip screw the compression spring continues to exert a force against the nut even after axial movement of the lag screw and nut toward the side plate. While the force exerted by the compression spring is reduced as it is extended, at least some of the compression force remains upon settling of the fracture in contrast to other compression hip screws.
One problem with the Charnley compression hip screw is that the barrel of the side plate has a relatively large diameter to receive the outer end of the lag screw, and both the spring and nut which fit over the lag screw. Since the barrel extends over the outer end of the lag screw within the proximal portion of the femur, a large amount of bone must be removed in that area in order to make room for the barrel. Many surgeons are reluctant to remove large amounts of bone in patients with poor bone quality due to osteoporosis or other conditions. In addition, the Charnley device provides no means for preventing the lag screw from rotating within the barrel of the side plate. As described above, the lag screw can eventually work its way through the femoral head, or move in the opposite direction into the soft tissue of the thigh, if it is allowed to rotate within the barrel.